In the control of systems having rotating drive shafts, it is generally recognized that torque is a fundamental parameter of interest. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades. Although great strides have been made, there remains a compelling need for inexpensive torque sensing devices which are capable of continuous torque measurements over extended periods of time despite severe environments.
All magnetoelastic torque transducers have two features in common--(1) a torqued member which is ferromagnetic and magnetostrictive, the former to ensure the existence of magnetic domains and the latter to allow the orientation of the magnetization within each domain to be altered by the stress associated with applied torque; and (2) a means, most usually but not necessarily electromagnetic means, for sensing variations from the untorqued distribution of domain orientations. The differences among the various existing or proposed magnetoelastic torque transducers lie in the detailed variations of these common features.
It is well known that the permeability of magnetic materials changes due to applied stress. When a torsional stress is applied to a cylindrical shaft of magnetostrictive material, each element in the shaft is subjected to a shearing stress. This shearing stress may be expressed in terms of a tensile stress and an equal and perpendicular compressive stress with the magnitude of each stress being directly proportional to the distance between the shaft axis and the element. The directions of maximum tension and compression occur along tangents to 45.degree. left-handed and 45.degree. right-handed helices about the axis of the shaft. The effect of the torque is to increase the magnetic permeability in directions parallel to one of the helices and, correspondingly, to decrease the magnetic permeability in directions parallel to the other of the helices. In their article "Magnetic Measurements of Torque in a Rotating Shaft", The Review of Scientific Instruments, Vol. 25, No. 6, June, 1954, Beth and Meeks suggest that in order to use permeability change as a measure of the applied torque, one should monitor permeability along the principal stress directions and pass the magnetic flux through the shaft near its surface. This is because the stress is greater, the further the element is from the shaft axis and it is along the principal stress directions that the maximum permeability change is expected. To accomplish this, Beth and Meeks used a yoke carrying a driving coil for producing an alternating flux in the shaft and pickup coils on each of several branches to detect the permeability changes caused by the applied torque in flux paths lying in or near the principal stress directions in the shaft. When the shaft is subjected to a torque, the mechanical stresses attributable to torque resolve into mutually perpendicular compressive and tensile stresses which cause the permeability in the shaft to increase in the direction of one stress and decrease in the direction of the other. As a result, the voltage induced in the pickup or measuring coils increases or decreases. The difference in magnitude of the induced voltages is proportional to the torsional stress applied to the shaft. A similar approach was taken in U.S. Pat. No. 3,011,340--Dahle. The principal shortcoming in these type devices is the need to accomplish permeability sensing along the principal stress directions with its attendant disadvantages, such as its sensitivity to variations in radial distance from the shaft, magnetic inhomogeneity around the shaft circumference and non-compensatable dependence on shaft speed. As a result, devices such as these have only found applications on large diameter shafts, i.e., 6-inches and larger, but have not been found to be adaptable to smaller shafts where the vast majority of applications exist.
It was felt by some that devices such as were taught in Beth and Meeks and U.S. Pat. No. 3,011,340--Dahle, wherein the rotating shaft itself acted as the magnetic element in the transducer, had significant drawbacks in practical application. This is because the materials and metallurgical processing which may have been used to impart the desired mechanical properties to the shaft for its desired field of use will, in most cases, not be optimum or even desirable for the magnetic qualities required in a magnetoelastic torque sensor. The random anisotropy in a shaft created during its manufacture, due to internal stresses and/or resulting from regions of differing crystal orientation will cause localized variations in the magnetic permeability of the shaft which will distort the desired correlation between voltage sensed and applied torque. The solution, according to U.S. Pat. No. 3,340,729--Scoppe is to rigidly affix, as by welding, a magnetic sleeve to the load-carrying shaft so that a torsional strain proportional to the torsional load is imparted to the sleeve. The measuring device employed now senses permeability changes in the rotating sleeve rather than in the rotating shaft. This permits, according to Scoppe, a material to be selected for the shaft which optimizes the mechanical and strength properties required for the shaft while a different material may be selected for the sleeve which optimizes its magnetic properties. As with prior art devices, the Scoppe torquemeter utilized a primary winding for generating a magnetic flux and two secondary windings, one oriented in the tension direction and the other in the compression direction. Although obviating at least some of the materials problems presented by Dahle, the use of a rigidly affixed sleeve creates other, equally perplexing problems. For example, the task of fabricating and attaching the sleeve is a formidable one and even when the attachment means is welding, which eliminates the bond strength problem, there remains the very significant problem that the coefficient of thermal expansion of the steel shaft is different (in some cases up to as much as 50% greater) than the corresponding coefficient of any magnetic material selected for the sleeve. A high temperature affixing process, such as welding, followed by cooling establishes stresses in the magnetic material which alters the resultant magnetic anisotropy in an uncontrolled manner. Moreover, annealing the shaft and sleeve to remove these stresses also anneals away desirable mechanical properties in the shaft and changes the magnetic properties of the sleeve. Furthermore, like the Dahle device, the shortcomings of Scoppe's transducer, due to its need to monitor permeability changes lying along the principal stress directions, are its sensitivity to variations in its radial distance from the shaft, magnetic inhomogeneity around the shaft circumference and dependence on shaft speed.
A different approach to magnetoelastic torque sensing utilizes the differential magnetic response of two sets of amorphous magnetoelastic elements adhesively attached to the torqued shaft. This approach has the advantage over prior approaches that it is insensitive to rotational position and shaft speed. However, it requires inordinate care in the preparation and attachment of the elements. Moreover, transducer performance is adversely affected by the methods used to conform the ribbon elements to the shape of the torqued member; the properties of the adhesive, e.g., shrinkage during cure, expansion coefficient, creep with time and temperature under sustained load; and, the functional properties of the amorphous material itself, e.g., consistency, stability. Still another concern is in the compatibility of the adhesive with the environment in which the transducer is to function, e.g., the effect of oil, water, or other solvents or lubricants on the properties of the adhesive.
In the article "A New Torque Transducer Using Stress Sensitive Amorphous Ribbons", IEEE Trans. on Mag., MAG-18, No. 6, 1767-9, 1982, Harada et al. disclose a torque transducer formed by gluing two circumferential stress-sensitive amorphous ribbons to a shaft at axially spaced apart locations. Unidirectional magnetoelastic magnetic anisotropy is created in each ribbon by torquing the shaft in a first direction before gluing a first ribbon to it, releasing the torque to set-up stresses within the first ribbon, torquing the shaft in the opposite direction, gluing the second ribbon to it, and then releasing the torque to set-up stresses within the second ribbon. The result is that the anisotropy in one ribbon lies along a right-hand helix at +45.degree. to the shaft axis while the anisotropy in the other ribbon lies along an axially symmetric left-hand helix at -45.degree. to the shaft axis. AC powered excitation coils and sensing coils surround the shaft making the transducer circularly symmetric and inherently free from fluctuation in output signal due to rotation of the shaft. In the absence of torque, the magnetization within the two ribbons will respond symmetrically to equal axial magnetizing forces and the sensing coils will detect no difference in the response of the ribbons. However, when torque is applied, the resulting stress anisotropy along the principal axes arising from the torque combines asymmetrically with the quiescent anisotropies previously created in the ribbons and there is then a differing response of the two ribbons to equal axial magnetizing force. This differential response is a function of the torque and the sensing coils and associated circuitry provide an output signal which is proportional to the applied torque. Utilizing substantially the same approach, in Japanese patent publication No. 58-9034, two amorphous ribbons are glued to a shaft and symmetrical magnetic anisotropy is given to the ribbons by heat treatment in a magnetic field at predetermined equal and opposite angles. Amorphous ribbons have also been glued to a shaft in a .+-.45.degree. chevron pattern, see Sasada et al., IEEE Trans. on Mag., MAG-20, No. 5, 951-53, 1984, and amorphous ribbons containing parallel slits aligned with the .+-.45.degree. directions have been glued to a shaft, see, Mohri, IEEE Trans. on Mag., MAG-20, No. 5, 942-47, 1984, to create shape magnetic anisotropy in the ribbons rather than magnetic anisotropy due to residual stresses. Other recent developments relevant to the use of adhesively attached amorphous ribbons in a magnetoelastic torque transducer are disclosed in U.S. Pat. No. 4,414,855--Iwasaki and U.S. Pat. No. 4,598,595--Vranish et al.
More recently, in apparent recognition of the severe shortcomings inherent in using adhesively affixed ribbons, plasma spraying and electrodeposition of metals over appropriate masking have been utilized. See: Yamasaki et al, "Torque Sensors Using Wire Explosion Magnetostrictive Alloy Layers", IEEE Trans. on Mag., MAG-22, No. 5, 403-405 (1986); Sasada et al, "Noncontact Torque Sensors Using Magnetic Heads and Magnetostrictive Layer on the Shaft Surface--Application of Plasma Jet Spraying Process", IEEE Trans. on Mag., MAG-22, No. 5, 406-408 (1986).
The hereinbefore described work with amorphous ribbons was not the first appreciation that axially spaced-apart circumferential bands endowed with symmetrical, helically directed magnetic anisotropy contributed to an improved torque transducer. USSR Certificate No. 667,836 discloses a magnetoelastic torque transducer having two axially spaced-apart circumferential bands on a shaft, the bands being defined by a plurality of slots formed in the shaft in a .+-.45.degree. chevron pattern, and a pair of excitation and measuring coil-mounting circumferential bobbins axially located along the shaft so that a band underlies each bobbin. The shape anisotropy created by the slots is the same type of magnetic preconditioning of the shaft as was created, for example, by the chevron-patterned amorphous ribbons of Sasada et al and the slitted amorphous ribbons of Mohri, and suffers from many of the same shortcomings. USSR Certificate No. 838,448 also discloses a magnetoelastic torque transducer having two spaced-apart circumferential bands on a shaft, circumferential excitation coils and circumferential measuring coils surrounding and overlying the bands. In this transducer the bands are formed by creating a knurl in the shaft surface with the troughs of the knurl at .+-.45.degree. angles to the shaft axis so that the troughs in one band are orthogonal to the troughs in the other band. The knurls are carefully formed by a method which ensures the presence of substantial unstressed surface sections between adjacent troughs so that the magnetic permeability of the troughs is different from the magnetic permeability of the unstressed areas therebetween. Inasmuch as the trough width-to-pitch ratio corresponds to the stressed to unstressed area ratio and the desired ratio appears to be 0.3, there is no circumferential region in either band which is intentionally stressed over more than 30% of its circumferential length. This very minimal stress anisotropic preconditioning is believed to be too small to provide a consistent transducer sensitivity, as measured by the electronic signal output of the measuring coils and their associated circuitry, for economical commercial utilization.
Notwithstanding their many shortcomings in forming sensitive and practical bands of magnetic anisotropy on a torqued shaft, the efforts evidenced in the Harada et al, Sasada et al, Mohri and Yamasaki et al articles and the USSR certificates represent significant advances over the earlier work of Beth and Meeks, Dahle and Scoppe in recognizing that a pair of axially spaced-apart, circumferential bands of symmetrical, helically directed anisotropy permits averaging axial permeability differences over the entire circumferential surface. This is notably simpler than attempting to average helical permeability differences sensed along the principal stress axes, as had earlier been suggested. Moreover, neither rotational velocity nor radial eccentricity significantly influence the permeability sensed in this manner. Nevertheless, these efforts to perfect means of attachment of magnetoelastically optimized material to the surface of the torqued member introduces unacceptable limitations in the resulting torque sensor. The application to the shaft of adhesively affixed amorphous ribbons suffers from significant drawbacks, such as the methods used to conform the ribbons to the shaft, the properties of the adhesive and the functional properties of the amorphous material, which make such ribbons impractical for commerical implementation. The use of rigidly affixed sleeves as taught by Scoppe and, more recently, in U.S. Pat. No. 4,506,554--Blomkvist et al, is unsuitable for practical applications due to the higher costs involved as well as the stresses created by high temperature welding and/or the uncertainties in magnetic and mechanical properties created by subsequent annealing. Likewise, reliance upon shape anisotropy or predominantly unstressed regions to create stress anisotropy present significant problems which make such techniques impractical for commercial implementation.
It is, therefore, apparent that despite the many advances in torque transducer technology, there still exists a need for a magnetoelastic torque transducer which is significantly more economical than previous torque transducers, allowing use in many applications for which such transducers were not heretofore either economically or environmentally viable, and which is applicable to small as well as large diameter shafts, whether stationary or rotating at any practical speed.